You have a trillion dollar/year military budget. You can buy all the toys (that are physically and financially possible to build with current technology). You can hire the manpower, command the airpower, harness the firepower, fire up the research labs. You've had years to prepare and train.

Before you stands the mighty city of Oppela, population (pre-war) about 4 million. Densely built, many high-rises, a metro system, a few wide roads with lots of back roads. It's been taken over by the evil bad guys (at least according to the massive PR-campaign by the media that you mostly control). There's very few civilians left (at least according ... well you get the idea). There's also tens of thousands (maybe 100,000) dug-in, well-supplied and well-motivated enemy fighters, many of whom have vowed to fight your forces to the death.

It should be pretty straightforward. Just go in and crush them. The only problem is that for all your military might, your home front is ridiculously sensitive to own-team casualties, and you're about to engage in the largest urban assault operations since Berlin (victory, but 350k casualties for attackers) and Stalingrad (defeat, 850k casualties for attackers).

Is it possible (using any and all available tech and the best tactics) to win this siege with near zero (own side) casualties? You can safely assume that you won't be able to talk your foes into submission (also, playing Justin Bieber songs really loud probably won't work either).

PS: No nuclear weapons. I have a severe allergy to nuclear detonations. They give me these rashes, makes it almost painful to shower in the morning.

PPS: Edited in from comments:

Q: Are there international or other laws you care to stick to?
A: I fired my legal team when I was elected to my high office. Nobody told me about any international treaties. If I don't know about them, they don't apply, right?

Q: Do you want the city itself to remain physically intact(ish), so that it can easily be repopulated by your guys, or are you happy for it to be flattened if need be?
A: City? What city? There was a city there? I don't know of any city. But if we're talking cities, we'll build a great, huge city. We build the best cities. It will be glorious.

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– Monica CellioAug 4 '16 at 1:37

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$\begingroup$Offer $10,000,000/year to each enemy combatant to become a peaceful civilian?$\endgroup$
– MichaelAug 4 '16 at 21:01

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$\begingroup$unless superman is there, metropolis should not be capitalized in the question$\endgroup$
– Steve CoxAug 5 '16 at 16:38

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$\begingroup$Was just thinking that you could probably reduce your problem immediately by introducing the mother liquor of trithioacetone into the city. You'd likely end up with a half-million stinking refugees.$\endgroup$
– PCARRAug 6 '16 at 20:27

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$\begingroup$With a trillion dollars, just save your money some time and buy the city?$\endgroup$
– Alexander von WernherrMay 5 '17 at 9:27

32 Answers
32

I think one of the key issues that you will face in winning this war/battle is if your own population does not fully support it. If your population is totally against losses from the military - I can only assume they are even more against civilian losses. The enemy is likely able to identify that your population maybe don't have the stomach for a bloody engagement. If they are in a no win scenario in a conventional sense they are likely to turn to asymmetric warfare/terrorism.

Unless you can rapidly prevent all citizens - civilian or otherwise - leaving the city and being able to travel globally. You may experience a home front terrorism issue. If you have been building up and preparing for years what has the enemy been doing? Sitting waiting? - doubtful. If they have any special forces troops etc they will probably be trained to blend into local populations. If you kill millions of peoples families you will radicalise huge numbers so you need to ensure containment. However if you can ensure containment then basically it is a perfect siege and 4 the million people will just starve to death.

Often this sort of resistance would take the form of civilian resistance in the city e.g. Maquis in WWII. However if they can't do that as there are no targets they would likely leave. Many soldiers left their own countries to form resistance units during WWII e.g. Czechs and Poles trained and armed in UK. They then returned to fight in their country e.g. Czech assassination of Reinhard Heydrich. Instead if if they can get into your country you have trouble - as they can form sleeper cells etc. and wait for the right chance. Depending on their cultura they may or may not be willing to launch suicide attacks but many would likely take huge risks to a carry out revenge attacks. Appropriate enemy culture may support suicide attacks as can be seen by Kamikaze attacks in WWII.

We don't have to look far to see how that works currently with groups like ISIS. ISIS encourage attacks from cities largely under siege but motivate people with their ideology to launch attacks against civilian populations in other countries. Terrorism against an already squeamish population would likely be highly effective.

Also are there elements of the enemy nationality as immigrants or even large number of 2nd generation immigrants within your country? Or even in nearby countries? America had Internment of Japanese Americans to deal with this threat during WWII.

Also do you fully control your own population? Are all elements of your own population fully committed to the cause? Depending on your tactics you may radicalise elements of your own population with no actual affiliation with the enemy. The risk would be media images of atrocities could sway your population against the action. This could increase the risk of homegrown domestic terrorism. Obviously propaganda works well here demonising the enemy - making them less than human. e.g. US Bug bunny cartoons of the Japanese used stereotyping to ensure the civilian population were more supportive.

So how do you win the battle for "hearts and minds"? Social media? Media blackout? I assume you can't enforce a media blackout and fully control the media otherwise the truth of your own causalities wouldn't matter? This may add another issue if the enemy is willing to stage fake atrocities to gain sympathy and generate support for their cause. Also look at political upheaval caused by anti Vietnam War protest in US in 60s. If the enemies only option is death - you probably can't win their "hearts and minds" so probably need to offer alternatives to prevent "Total war".

If the enemy is backed into a corner and conventional warfare won't work and surrender is not an option then they are likely to try these approaches.

$\begingroup$The only option for the win is complete racial genocide and subsequent scrubbing of the history books. And at that point, did you really win?$\endgroup$
– Jammin4COAug 3 '16 at 15:44

$\begingroup$Yeah guess bio weapons targeting specific genes shared by target race on a global scale is a tech option. Not exact by any means. Of course we are talking city but assuming racial homogeneity. Again assumes your own population supports genocide. Trying to wipe out a globally dispersed race would be all but impossible. Even identifying. Also mixed race children and other nations probably wouldn't support it.$\endgroup$
– GraemeMillerAug 3 '16 at 16:00

$\begingroup$Sorry, I was generalizing. I didn't mean a race by genes. I meant the people group that is besieged.$\endgroup$
– Jammin4COAug 3 '16 at 16:06

$\begingroup$@Jammin4CO, what you'll notice is that most of the other answers involve total genocide and scrubbing of the history books$\endgroup$
– SeparatrixAug 4 '16 at 10:51

$\begingroup$Objection - The president at the time, F[reaking] D[umb] Roosevelt is largely responsible for those internment camps which are very unconstitutional. Carry on.$\endgroup$
– Xandar The ZenonAug 17 '16 at 4:25

Drone warfare

Your people might not be disposable but you have the money and you have the toys. There's no reason to actually send any of your people within 500miles of the place. So you lose a few drones, it doesn't matter if you're losing a drone per enemy kill, it's only money and you're not losing people.

As a tactic you don't even need to be subtle, just send two more drones in for each one you lose. Think doodlebugs or WWII Russian infantry. Do it by sheer weight of numbers, flying, wheeled and walking.

Little 8 legged crawling bombs that could walk up walls, into tunnels and pipes, sneaking into every little nook and cranny would be a morale killer for defenders. Imagine in your most private moment, pants around your ankles and a bomb comes crawling out of the pipe underneath you, Oppela is not going to be a happy place.

Build a high wall around the city.

Then flood the entire city with $CO_{2}$.

This should displace all the oxygen in the city and being heavy, stick around of a long time while also flowing down into the deepest hideouts. Though toxic at high levels, normal diffusion will clear the city and make it safe again after you take down the wall.

The only place where people would be safe would be in the upper floors of skyscrapers, where they are very vulnerable to your more conventional weapons, unable to move around and if you cut off electricity and water, they will not last long. Some of them could be convinced to surrender if you want the PR value, though beware of booby traps and suicide bombers.

An extra bonus is that no combustion engine will work without oxygen, so vehicles become useless, as do diesel generators. If some sneaky people try to run a generator up high in a skyscraper, you can easily spot the infrared signature and send a missile to turn it off.

After the city quiets down, you will need some drones with heartbeat detectors to locate any bunkers that had better air recycling than most.

Building the wall

This is the riskiest part of the plan, though remote controlled construction vehicles can do most of the work. Start with reinforced checkpoints. Make the first ones look vulnerable but fill them with nasty surprises for any attackers from within the city. It should be quite clear that you are trying to lure them out into the open to be cut down.

Leave some supply routes untouched for as long as possible. Call your project a "Containment Zone" and declare how it will keep the rest of the country safe from those evil marauders in Oppela. Finish the encircling before building it any higher. Put sensors in the ground everywhere and collapse all tunnels except near the supply tunnels the enemy uses.

When you are ready, raise the walls high. Declare your containment zone a success publicly. Then put them all to sleep. The gas itself would be unnoticeable to the entrenched fighters unless they are specifically monitoring it and once it reaches dangerous levels, they will not be clearheaded enough to mount a coordinated response.

The benefits of this tactic are that you are not left with dangerous chemical, biological or nuclear materials that might escape the quarantine zone. Also, you don't do a lot of damage to the city itself. The small amount of fighting that's left will have you in a position of overwhelming advantage, either at the wall if they try to break out, or with the enemy trapped in skyscraper, unable to leave.

The downsides are that wall builders may be attacked by an overwhelming force in the early stages, that you need time to generate enough $ CO_{2} $ and of course that you are committing horrific genocide, you evil bastard.

Note: I feel dirty just suggesting this tactic and I'm happy this would not be feasible with real-world monetary, resource, political and ethical constraints. However, real-world conflicts have shown us that conquering a city is an extremely costly, bloody and wasteful affair. There simply is no way to keep the civilian population safe and still find and defeat the armed defenders without going into the city and exposing yourself to all they've got. Even after the city "surrenders", individual fighters may pop up and kill a bunch of your people by suicide bombing. Hence the radical decision that no casualties means no potential fighters left.

$\begingroup$this is a better answer than any of the other “poison gas” answers, since it is not enough to just filter out CO2, and it will kill anyone no matter the hideout given enough time$\endgroup$
– taylor swiftAug 1 '16 at 15:29

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$\begingroup$This would work great, except for the part where any metropolis 4 million strong has hills and stuff.$\endgroup$
– Serban TanasaAug 1 '16 at 15:40

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$\begingroup$Use a different flood medium like good old fashioned water. Then send in weaponized ROVs to bust any bunkers and flood those as well. When all the thermals come back nice and cool, pull the plug and send in a few more drones for any possible mop up that might be needed.$\endgroup$
– Steve MangiameliAug 1 '16 at 16:09

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$\begingroup$they would try to stop you building your wall, there are 100,000 armed people in there, they'd manage to kill at least 1 of your builders.$\endgroup$
– RacheetAug 1 '16 at 18:45

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$\begingroup$@Martijn a city of 100,000+ inhabitants cannot possibly grow enough food for itself if it didn't already have advanced hydroponics towers or something. In fact, a fully populated modern city would run out of food in a day or two. The reason to leave open the supply routes is exactly to prevent a desperate mass assault on the wall until it's too late.$\endgroup$
– CyrusAug 2 '16 at 8:45

Use economics. You have a trillion dollar defence budget, a risk adverse political class, and a massive PR/propaganda machine, why not get them to fight themselves? Offer a million dollar bounty for the head of each of the 100,000 hardcore committed troops your are fighting against. Yes 100,000 heads of your enemy at $1 million a pop would equal 1/10th of your defence budget but all of your troops gets to go home to mom.

Plus if you really want to Machiavellian about it many of the people you would need to pay off for the dirty work you got them to do would probably be killed off in the mayhem that follows so you wouldn't need to pay them.

Also you could use your giant PR machine to turn the siege into the "greatest reality TV show of all time," and sell advertising space. I'm sure countless defence contractors would beat down your door to sponsor your war.

$\begingroup$Similar to the "use mercenaries" answer previously, but a very clever take on the problem for sure.$\endgroup$
– JBiggsAug 1 '16 at 19:11

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$\begingroup$Seems to work not so well for the US. How big was the bounty on Bin Laden?$\endgroup$
– Serban TanasaAug 1 '16 at 21:01

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$\begingroup$HUNGER GAMES IS A LEGITIMATE WARFARE STRATEGY$\endgroup$
– Mirror318Aug 2 '16 at 4:55

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$\begingroup$One problem with bounties (as has been seen in Afghanistan, I believe) is that the people who claim them aren't always fussy about whom they deliver.$\endgroup$
– Anton SherwoodAug 2 '16 at 7:47

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$\begingroup$@AntonSherwood That's right, but as I read it, as long as the head in that bag did belong to someone within the city it fully qualifies for the bounty.$\endgroup$
– JimmyBAug 3 '16 at 11:28

You may be overthinking this one. Let's look at the Gulf War as a contemporary example. The allies deployed 950,000 soldiers against Saddam's 650,000. 292 allied soldiers were killed (half of them by accidents), and up to 35,000 Iraqis were killed. As things go, the trend is towards less deaths due to combat over time owing to technological developments. The 2003 invasion of Iraq saw the US led coalition deploying 380,000 soldiers with 172 deaths. It may not be zero, but considering the numbers involved, it's an incredibly low figure.

Casualties depend on how evenly matched your forces are with your enemy, and how determined they are to fight. This wasn't just a military conflict; the Americans in particular led a propaganda campaign against both the Iraqi soldiers they were about to fight, and their own people. That would be necessary in your case too. It has been said that America's propaganda, inclusive of leaflet drops, were a contributing factor to the surrenders in the Iraqi army.

Another case study worth considering is Operation Cast Lead. This is a good example of the evolution of hardware and tactics to suit urban fighting. And you may like to examine recent innovations from the Israeli defence industry, which are often specifically for circumstances like these; and sometimes it's the little things; like having a gun which can shoot around corners. However, in this case (unlike your own) there were many civilians present and killed.

In your situation the fanatic enemies may deliberately cluster around what's left of the civilian population, which causes a lot of problems for even the most sophisticated guided bombs. Modern artillery shells can be guided to within meters of their targets when fired from tens of kilometres away, and so if the enemy forces are nowhere near civilians they can quite safely and accurately be shelled into oblivion at great distance with artillery, aircraft, and missiles.

Perhaps the waiting game is the best option. Allow any civilians safe passage out, hold the perimeter around the city, turn off the water, and wait. At best it'll be a few months until the defenders are dead or too weak to fight. If they do try and break the siege they'll be advancing into your traps and defences. That would depend on your ability to create and hold a perimeter, assuming they no longer have supply lines.

The bottom line is that with patience, overwhelming numbers, and superior technology, you can win with very few casualties.

$\begingroup$Very interesting. Could you provide a few links for the "recent Israeli innovations" you mention? Also, I recall the US taking of Fallujah (a much smaller city) in Nov 2004 resulted in 700 US casualties (dead and injured).$\endgroup$
– Serban TanasaAug 1 '16 at 16:48

$\begingroup$The second battle of Fallujah is a good counter example yes. Those I spoke of I referred only to deaths, which may have been short sighted. But it's coffins being carried home which cause the greatest impact psychologically since they are a direct link between the war and its nastiness. Injuries and psychological damage, whilst real and horrible, are less of a concern for PR (unless occurring in the scale of soldiers wounded as say, gas in the first world war).$\endgroup$
– inappropriateCodeAug 1 '16 at 17:02

$\begingroup$With regards to Israeli innovations, was primarily thinking of cornershot, for its use in commando raids and street to street fighting. But @SerbanTanasa bested me, it seems!$\endgroup$
– inappropriateCodeAug 1 '16 at 17:03

From the question, civilian enemy militant causalities are not a concern.

this calls for a multi phase operation.
Phase one, remove their ability to communicate. Deploy Electronic warfare units around the city and jam all radio frequencies. Destroy all of their radio stations, tv stations, internet hubs etc.. You wouldn't want these civilians soldiers to tell the truth of atrocities being committed against them lies and false propaganda in attempt to hurt the morale of your troops. Then, Take out railroads, roads and ports and airports in the city. You want to deny the population the ability to get supplies in and transport people to outside of the city. You don't need to completely cut the city off from the outside world, but, if you prevent the transportation of large amounts of supplies you will have hurt the city pretty badly.

Phase 2. Use aerial strikes to eliminate all medical care facilities: hospitals, clinics, dentist offices, manicurists, barbers. You get the idea. Destroy their water treatment plants, power stations, gas lines, sewage facilities, recycling centers, incinerators. This will cause sewage and garbage to slowly fill the city. They will have major sanitation issues and be ripe for plagues.

Now this defenseless city bastion of the enemy is going to have serious issues, all of this damage will either break t he enemy civilians resolve, or, it could strengthen it. (a rare and unfortunate side effect)

Phase 3. Deploy your main weapon! There is this lovely disease called the black plague, it has about a 30% casualty and whats more is there is a vaccine for it! Unfortunately for the residents of Oppela most people aren't given this vaccine. So these poor residents will have to depend on getting it from their destroyed hospitals. You can expect high casualties from this attack and, protect your forces from this dangerous disease. Then, hit them with the bird flue, typhoid fever, dengue fever infested mosquitoes. The few doctors who survived your purge will be insufficient to cope with this onslaught of diseases.

The final phase of this operation is mop up. You should launch this attack about 3-5 days after you infected the population with dengue fever, it causes extreme muscle fatigue meaning any enemy civilians combatants will be unable to run away fight effectively.

A lot of the others mentioned problems that you could have with PTSD and after action suicides in your military forces. I don't know if there is an effective way to handle this, but, you could overcome the negative effects this would have on your civilian population with the right propaganda. Videos of mothers carrying infants and attempting suicide bombings against your troops. 10 year old soldiers charging entrenched positions with assault rifles. This would also help paint a picture about how evil the enemy is.

$\begingroup$Better have some accurate targeting system on that moonrock. Does that fit within my trillion dollar budget?$\endgroup$
– Serban TanasaAug 1 '16 at 17:09

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$\begingroup$@SerbanTanasa Oh, it should be fairly easy to implement very precise ballistic calculations. And yeah, I'd say so! And about costs, check this link: minorplanetcenter.net/blog/…$\endgroup$
– OnoSendaiAug 1 '16 at 17:12

$\begingroup$@SerbanTanasa quote: "[...] the feasibility of retrieving a 7m asteroid and bringing it to Lunar orbit estimated the cost at $2.6 billion." That would be 368 tons of astro-fun, assuming a somewhat light density of 2.05 grams per cubic centimeter.$\endgroup$
– OnoSendaiAug 1 '16 at 17:15

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$\begingroup$@SerbanTanasa Surely, enough to atomize small, un-patriotic pebbles. But a baby of that size would survive some ablative action.$\endgroup$
– OnoSendaiAug 1 '16 at 17:20

Chemical Warfare

Drones as mentioned by @Separatrix are an excellent way to minimize your casualties, but are ineffective when people create tunnels, adapt to living in caves, and start a guerrilla style warfare if and when you choose send boot on the ground. Your first and foremost job would be to surround the city, to minimize smuggled resources, forcing people to rely entirely upon only available resources.

Disrupt or destroy electric grids (Artillery/Bombs), water supply (through poisoning), hospitals and any government facilities which would contain gas masks, or medicines. Following a co-ordinated heavy artillery and drone shelling in the midst of ensuing chaos, use either aircraft bombs or binary munitions to completely cover the entire city with Nerve Gas or Asphyxiate. You also have a wide variety of chemicals to choose from. Most are odorless, colorless, and practically undetected till effects start. Most notable ones are Sarin, Novichok, and VX (V Series).

This not only ensures high number of causalities, crushes their structure but also, the sight of mutilated corpse with blisters, and agony of the affected but alive population does irreparable damage to their moral, allowing for a much easier surrender. If you are still unsure about it, even Hitler choose not to use mustard/Chlorine gas in WW2 after seeing its horrifying effects in WW1.

A metropolis is inherently vulnerable to interdiction, if you blockade all the supplies, drop a 500 lb bomb on the water treatment plant/pipelines. Then wait for them to surrender or starve to death. If they have their own food, yummy yummy mil-rats, they will still suffer dehydration with no water.

People tend to forget that modern cities are not self sufficient, they rely on a steady input from food production areas (farms) and water processing: interrupt that and everybody dies. (and not quick and clean either)

A military installation might have its own hardened water supply, but probably doesn't. They also might have sufficient supplies of shelf stable foodstuff that everyone hates. But most military would need a really good reason to invest in such things, and a good attacker won't give them a lot of advance notice. So what you are left with is IF there is a military base inside the metropolis (most are not) then it might have water available, otherwise you can kill them by denying water/food.

I think it depends on the time scale that you want to re-take the city in. A medieval siege was usually conducted because the casualties that would happen in a frontal assault were not deemed acceptable to the attacking force.

In addition to really loud Bieber (Might as well try) you just stop all incoming food and services from a distance far enough away to be out of combat range of the defenders. If you can lob in some rockets, bombs, artillery from that safe distance (including very high level planes in this) then add that into the mix as well.

Eventually the defenders will starve, get bored, die of old age without you taking a single offensive casualty. It may however take some time and they will probably destroy most of the city themselves before you get it back.

$\begingroup$@Serban: canned goods only last if you don't eat them, and 4 million people get through a lot of baked beans. Most cities don't have a whole lot of stockpiled food, call it lean logistics.$\endgroup$
– Steve JessopAug 1 '16 at 15:52

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$\begingroup$Most cities only hold enough food to last the population a few days or weeks. If you can precision bomb all internal warehouses and cut off all supply lines then the population will start to have a hard time pretty quickly. Without incoming food or water, power to cook it, diesel to power cars/tanks it gets uncomfortable quite quickly. Also removal of medical facilities and sewage removal and waste reprocessing will make living standards drop. I didn't say it was very quick but it's not putting a single person at risk which is the main driving factor.$\endgroup$
– Paul7926Aug 1 '16 at 16:07

You may want to define "winning".
I assume you want to own this metropolis, and not just own a really impressive pile of junk.
I take it that you are not interested in survivors (the kind of stories they keep telling can be very upsetting for the tender and fragile souls of your people on the home front, and noone wants that, right?)
So, i would say that carpet bombing, in spite of all the fancy visual effects it provides, is not an option (also, it's simply not hipster enough)
Using gas or biological weapons sounds like a good idea, at least as long as your weather forecasts are reliable enough to predict wind directions for a few hours.
Unfortunately, gas masks are a thing, and while you could deminish the enemy forces over time, you simply cannot get them all in one go.
Separatrix already suggested the coolest option: 8-legged bombs walking up walls.
So i will use a much more cynical and direct approach:

Just hire mercenaries.

They are not your own people, so the home front won't care.
Since you have the money and the toys, hiring them, offering a substantial bonus for secondary goals (like leaving things standing, doing some basic cleanup of leftover enemies, and whatnot), and of course supplying them with all the fun toys your precious military industry provides, you should be all set. Especially if you are not in a particular hurry.
As a bonus, if things go haywire, you already have someone to blame.

Obviously, that's still not particularly hipster.
But we can fix that, by supplying them with power armour, at least officially attemting to make them bullet proof, and, the most important part: provide them with biodegradable ammunition!
This will clearly tell your sensitive people on the homefront how really really nice your war is!

$\begingroup$We tried the merc route, the downside is that the evil bad guys kept beating them and taking away their toys. Hmm, perhaps we need to train them better than "here's this rifle and this crate of expensive military equipment, go attack that way"...$\endgroup$
– Serban TanasaAug 1 '16 at 15:38

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$\begingroup$Most modern mercenaries are veterans of active conflicts with plenty of combat experience. Check out "The Modern Mercenary" by veteran merc Sean McFate for a riveting analysis of how fighters-for-hire are actually the historic norm and the standing armies of the post-Westphalian order are actually the historic exemption.$\endgroup$
– rm -rf slashAug 1 '16 at 17:48

2

$\begingroup$Reminds me of this scene... Zorg: I hate warriors, too narrow-minded. I'll tell you what I do like though: a killer, a dyed-in-the-wool killer. Cold blooded, clean, methodical and thorough. Now a real killer, when he picked up the ZF-1, would've immediately asked about the little red button on the bottom of the gun. [Scene shifts to Aknot, who is staring in confusion at the little red button. He shrugs and pushes it] ... Bang!$\endgroup$
– SeedsAug 1 '16 at 19:57

Convert your trillion dollar budget into pennies. According to the megapenny project, one cubic foot of pennies is about $500.00. So for a trillion dollars you end up with 2 million cubic feet of pennies.

Then in the name of peace, "donate" all the pennies to the city. Donate them with bombers, with catapults, with shotguns and sling shots. Even if 2 million cubic feet of shiny, slippery coins, delivered at velocity, is not enough to kill and bury the entire population of the city, it is enough to block up a lot of doors and roads, fill up the sewers and generally make life miserable for the bad guys.

Then, one year later, when your military budget renews... do it again!
I guarantee that within a decade or two, you won't be able to even see that city under the enormous pile of coins.

And as a bonus, once you've destroyed your enemy through pennicide, you can re-enter the no longer defended city and take all your pennies back!

$\begingroup$Of course then they'll have the money to buy reinforcements... Basically declare that whoever liberates their city can keep the pennies - and in case you think that they won't be able to buy anything with a ridiculously unmanageable amount of change remember that the copper in a penny is worth more than a penny! You'll have literally given them over a trillion dollars worth of copper as well as exhausted all of your own military budget.$\endgroup$
– colmdeAug 2 '16 at 11:28

$\begingroup$Oops, misread google - copper and zinc but still worth slightly more than a penny apparently$\endgroup$
– colmdeAug 2 '16 at 11:36

$\begingroup$@karenorin, My answer was offered as tongue-in-cheek commentary on the idea of a trillion dollar defensive budget. In pennies, that is a column which is ten feet wide, ten feet deep and nearly 4 miles high. It is an unimaginable amount of money and to waste it on destruction is a whole new level of wrong. All that being said, your idea of simply burying the city in mud would probably work and would be much cheaper than (but not as cool as) my suggestion.$\endgroup$
– Henry TaylorAug 5 '16 at 5:49

$\begingroup$I suspect price of pennies would go up as you implemented this!$\endgroup$
– crobarAug 5 '16 at 11:58

You don't need nuclear weapons to cause an absurd amount of damage. You'll obviously need more bombs, but there's nothing that a good carpet bombing can't solve (except of course if your problem isn't about destroying stuff).

Bombing is a tried and tested way to level a city. It just works. But it does take some time. It might give some people some time to evacuate underground. Underground is significantly harder to reach with bombs, though if you succeeded in driving all enemies from the surface, that's a definite step in the right direction. They're trapped, they won't be able to do much (unless they are crazy prepared with secret underground bunkers full of weapons and access to super-secret defense system that is bomb-proof, but that's a rather small probability).

I'd suggest thermobaric bombs, which are the closest you can get to a WMD without being one. Thermobaric weapons put the emphasis on the heat and shock waves, much like a nuclear weapon really but without the radiation. That will be super effective against all building types.

Also, a little technical fact for you, thermobaric weapons are alternatively called "fuel-air", because they consume oxygen. Such a weapon in an enclosed space can do quite some bit of damage by effectively sucking all the air. So even underground, people aren't safe from those.

Biological weapons, gas, and other banned weapons

Enclosed space is also a good territory for biochemical weapons. If you can seal subway entrances (hint, you can use bombs for that), it's only a matter of pumping enough gas inside and guarding all known exit points. That obviously presumes you have bombed the surface first and people have fled underground.

You could instead just poison the water system (and the gas system, why not). However, I personally advise against that. I'm not a fan of biological warfare. You never know when bio weapons will come back to bite you in the gluteus maximum. Gasing people underground is relatively safe, but a virus in the wild is uncontrollable. Especially if the virus is designed for maximum casualty. But hey, it's your war.

Random acts of terrorism, electronic warfare and general chaos

Some people just want to watch the world burn. Chaos is a powerful weapon. At worst, it can help soften the enemy before you strike which is always useful.

If the real world tells us anything, it's that terrorism can successfully divide people. You just need exactly one bad apple to prompt some people to declare a whole ethnicity to be a bunch of terrorists, and I wish that was exaggerating. And you know what they say, divide and conquer. You just have to take it literally.

You can also create social tensions and divide people in other ways, but they are less immediate and usually a byproduct of decades of (in hindsight) bad policies. That last one implies access to your enemy's decision-making process, which means at that point there are more effective things you could do (like petition for secession, start a war with someone else, etc.).

Electronic warfare is another good approach to creating chaos, and really all you need is to cut the power. A simple blackout is more immediate than a long propaganda warfare, and it has the added advantage of denying your enemy access to electricity.

Hm, are you guys forgetting something? There's no time limit on capturing the city, so - starve them out. It's a metropolitan area with probably 0% arable land and 4 million hungry mouths to feed. Bomb their wells, cut off their water and food supplies, and give them a clear route to escape - they'll leave on their own after a year or two. Make sure you use mostly captured soldiers and civilians from their side to clear the booby traps.

Warning, I am going to mention some really gross stuff here. I will not post pictures. You may probably want to google for images. Don't do that if you cannot stand some really disturbing gory images.

You are going to need:

A few dozens of elite spies;

Access to a number of hospitals, veterinary clinics, biology labs, and the cities water supplies and treatment plants;

Large amounts of the rabies virus;

Tons of poisoned Krokodil.

Krokodil is the key here. It is a powerful analgesic, and also a very addictive drug. But you don't want it for these features. You want the cheaper, street variety of the drug, which causes skin and muscles to rot and fall off the body. This drug can do so in a very dramatic ways, which has caused some people to call it 'the zombie drug'.

Innoculate some hospital patients with rabies first. Do so for a few months. Let doctors diagnose people with it. Let the media know that there are mysterious, unexplained cases of human rabies happening to people eho had no contact with animals.

This may cause some small amounts of hysteria, and people will want to be vaccinated. You've got to sabotage their vaccines supplies. Be stealthy - make it so that people are only being injected with Saline IV.

Next, pick a few hospital patients and kidnap them. Take them to labs. Apply Krokodil and other drugs on them and use psychological torture on them. Make them violent and broken. Starve them. Then release them, few ones and days apart at first, in the slums o the city. Also gore and kill some innocent people in the slums to make it look like your Krokodil victims killed them.

Bonus if the Krokodil victims have body parts of other missing people in their stomach when you release them.

There, the city now thinks it has a zombie plague happening to it. Drive the hysteria up a notch by poisoning the water supplies with the poisoned Krokodil. Innoculate stray animals with rabies, and home pets in clinics too.

Scale this up until people start fleeing the place. At some point it will become a ghost city, with only a few thosand people still living in it. They should all be either scavengers or that kind of people who build bunkers to survive for years isolated from the world. These guys will be less than a skeleton force when you finally send your military in, probably disguised as an international zombie investigation and cleanup task force.

$\begingroup$So, this is basically "scare people with rabies & meth heads that look like zombies"? Sounds like some bad neighbourhoods in real world cities now...$\endgroup$
– Xen2050Aug 2 '16 at 12:22

Assuming you have the time, you may starve the city into submission or 'poison the well'. But surely these have been considered?

I noted that you mention a severe allergy to nuclear weapons. I'm wondering if perhaps you will be somewhat less allergic to the nuke's ugly cousin?

Mr. Neutron Bomb.

There's only one way to find out! He's still out there, somewhere (supposedly banned by treaty?)

He'll make short work of Opella's population, and leave the city infrastructure completely intact (except the electronics, which may become slightly fried). Your army should be able to walk in there within a few weeks. Not even the cockroaches survive.

$\begingroup$A neutron bomb is simply a nuclear device with different yield settings, and is expressly disallowed in the question.$\endgroup$
– March HoAug 2 '16 at 2:23

$\begingroup$@MarchHo I would classify a Neutron Bomb as nuclear, but I wouldn't classify it as 'simply a nuclear device with different yield settings'. It is a different beast altogether which I decided to mention anyway - fully aware of the question. It's just that, with allergies, sometimes you may be allergic to this cat but not that cat, if you know what I mean?$\endgroup$
– PCARRAug 2 '16 at 16:08

$\begingroup$@MichaelKjörling Citation certainly is needed! I'm not finding much material to support the statement, either. In fact, I'm seeing some trends towards the opposite! I will edit my answer accordingly until I find something concrete.$\endgroup$
– PCARRAug 2 '16 at 16:11

$\begingroup$@MarchHo, I suggested this in the comments and it is curious as how the OP has not addressed any of the other 7 references to the use of radiation as the weapon, which tells me the OP has no retort. IMO, our answer is the most logical given the parameters by the OP. While a neutron bomb may be nuclear, it isnt in the traditional sense (how I believe the OP envisioned) and depending on how detonated, wouldnt even leave radiation afterwards.$\endgroup$
– Matthew PetersAug 2 '16 at 22:12

Just buy the city

Alaska comes to mind here, once owned by Canada, it was then sold to Russia and then to America. Now this is a land mass rich in oil and was sold finally for a total of +4 billion. If I have 1 trillion dollars and only 4 million citizens, I can buy the city from the mayor and rise the rent to the point where the remaining 3999999 citizen just move rather than pay $100000 a month for water.

$\begingroup$I don't believe Alaska was ever "owned" or sold by Canada, the Russians were there (according to (Wikipedia)[en.wikipedia.org/wiki/Russian_America] from 1733 to 1867 (to just before the modern "Dominion of Canada" existed), and at some point the Spanish were busy naming places like Valdez & trying to claim the Pacific NW (maybe everything touching the Pacific Ocean?).$\endgroup$
– Xen2050Aug 2 '16 at 12:16

You can try the old fashioned approach. Why bother with fancy new weapons technology. Stick to the tried and true. Fire bombing. With your budget you can afford squadrons of heavy bombers to unload thousands of tons of napalm and phosphorus bombs on the city. Today we better meteorology, geodetic and terrain mapping and computer modeling it should be possible to know where to drop the bombs to ensure the maximum probability of a fire storm starting and spreading through the city.

Will this work? Of course it will, remember Dresden and Tokyo. There were post-war studies reviewing the effectiveness of old-style aerial bombing. They can be used as a planning manual for this operation.

This strategy will soften up the Bad Guys occupying Oppela. Tank divisions can be sent in to mop the rest of the demoralized population and finish off the remaining soldiers.

$\begingroup$Do we even have the capabilities to firebomb entire cities? I mean, fleets of thousands of B-2 bombers might bust my trillion dollar budget. Those suckers are expensive.$\endgroup$
– Serban TanasaAug 2 '16 at 18:36

$\begingroup$Victory never comes cheap. If necessary, you could build older model bombers like B-17 and B-29's, with turbo-prop engines, to deliver the bombs. They might even be one-time use bombers. You only need air superiority to ensure its success. Also, if you surround the city incendiary warheads can be delivered by rockets and artillery.$\endgroup$
– a4androidAug 3 '16 at 2:50

Nuke it from orbit.

It's the only way to be sure.

This might be a pretty boring answer, but if your obstacle is a city, and removing the city would give you success, then that problem is fairly easily solved. Bombing cities has been the best way to destroy cities for a long time now- why change the status quo?

Yes, yes I see

PS: No nuclear weapons. I have a severe allergy to nuclear detonations.

So conventionally bomb it from orbit. (A much less catchy and memetic headline though.)

$\begingroup$Embarrassingly, I had to look up that detail about the Battle of Stalingrad just now. It seems that the Nazis didn't have enough technology or resources to continually bomb the city till there was nothing left. I had imagined destroying the city so much that there is no economical reason to ever try rebuilding. Hard to survive and hide in rubble like that.$\endgroup$
– TyrannosaurAug 1 '16 at 16:05

Start by airdropping propaganda pamphlets and posters urging the people of the city to lay down their arms and resist the corrupt rule of the elites who brought this conflict upon them, blah blah Edward Bernays blah.

If the effort does not succeed - as it will likely not - schedule a carpet bombing run that fully encircles the city with a one-mile radius. After the ring of fire is dropped, demand unconditional surrender.

At night, send (hopefully automated) non-military vehicles (such as the Red Cross) to take as many civilians away from the city as possible. Make the noncombatants think of the conflict as their rulers' fault rather than an ultimate fight to the death. If you have drones, use them to prevent enemy combatants from commandeering the vehicles or preventing civilians from escaping.

The next day, repeat the ring of fire with a one-half-mile radius. The next night, repeat the Red Cross run.

The next day, repeat the ring of fire with a one-quarter-mile radius. The next night, repeat the Red Cross run.

The next day, send as many vehicles as possible and economical to take civilians away from the conflict. Spend the entire day doing this, from dawn till dusk.

Once the sun is down and the sky is black, repeat the ring of fire with zero radius. Continue each ring of fire every hour with decreasing distance from the city center.

At daybreak, resume the evacuation operation. At night, resume bombing.

In the end, you either get surrender, or a city blown to smithereens. Either way, you troops can go home.

$\begingroup$Or we expend most of our budget to hack into the matrix, SSH into the box that manages the city, and run your username as a command.$\endgroup$
– RangerAug 1 '16 at 18:04

$\begingroup$Not to mention the soldiers might get some of your Red Cross vehicles, but you did say they could always automate them. Still I don't know how many people would willingly jump into a robot car sent by the "enemies"$\endgroup$
– AreebAug 1 '16 at 19:10

$\begingroup$Well, I recognize that enter the city could be a problem xD. My idea would work best in a Western country.$\endgroup$
– DanielAug 1 '16 at 17:05

$\begingroup$Ownership only works when enforced. While you haven't conquered it, there's no reason why the existing government can't simply nationalize all those core services after you bought them, or why the enemy fighters can't dedicate a bunch of armed bodyguards to the leadership of all these services and ensure that they do what needs to be done instead of obeying the "owner".$\endgroup$
– PeterisAug 3 '16 at 11:27

I'd have suggested a neutron bomb, but was beaten to it by PCARR.
Second choice, a satellite in synchronous orbit that beams down powerful X- or gamma-ray radiation. This will induce radiation sickness in anyone in the city that is in the open, or inside non protected buildings.

The satellite can be protected against anti-satellite rockets launched from the metropolis by AMMs under your control near the metropolis. Or from other satellites with AMMs, but watch out for the resultant space junk.

Third choice:
Seed the city with radioactive materials (preferable neutron emitters), with a half-life of around a day. This will kill (or at least seriously sicken) anyone exposed to it, but after a month, the radiation will drop to background levels, assuming that the decay products are not radioactive themselves.
Finding a suitable material is left as an exercise for the interested reader.

$\begingroup$Welcome to the site, wizardzz. Please note that the Worldbuilding SE is dedicated to specific questions and answers. Answers are expected to include details, rather than be one-liners. If you would edit your post to include additional details, the community would appreciate it. Feel free to take the tour to get a better understanding of the site.$\endgroup$
– FrostfyreAug 1 '16 at 15:54

The simplest way is pretty boring: just starve them out. Cities have very high population densities - a lot of people in a little space. Cities don't generally produce any of their own anything, especially food, and the supply has to be extremely high to keep up with demand. Put checkpoints on all major highways and stop trucks from entering the city. Most grocery stores have to be stocked weekly or they'd run out of food just from regular business.

Two weeks into the blockade, the population of 4 million will turn on that little group of 100,000 for their supplies.

Bonus: No real structural damage will be caused to the city buildings, aside from a few riot fires, and if you time it right you could (optionally, as an evil overlord) save the majority of the civilians, which is good PR.

My dear leader, If you will allow me the great honor of serving our glorious nation, I will, with somber heart and determined mind, rid this planet of the scourge that is Oppela. I will begin with the judicious deployment of bunker busters in a grid pattern at quarter mile intervals across the entire region. This will open the evildoer's tunnels and dug-in positions to the air. Oh, what wonders we have in air. Your loyal servants will then follow with numerous vacuum bombs, fuel air blasts, thermobaric weapons! OH THE SPLENDOR! Yes, it will be WOUNDEROUS!

For good measure, we'll dump thousands of gallons of diesel fuel on the rubble and ignite it.

Once the rubble cools, your great city is ready to be built. A city worthy of your greatness, oh leader!

On second thought, perhaps my colleagues over at Evil Genius Community Hospital can formulate a salve for that rash!

There are a number of ways to go about it, most of them are simply old school mass destruction, with the suggested methods like:

Burn it all (firebombs/napalm for example)

Drown it all (a wall around the city, then fill it with water)

Irradiation to kill everything

Biological warfare

Chemical warfare

They all basically do the same, but in different ways.

Then there's time-based stuff, like waiting them out, they will starve eventually, but that may be problematic if there are men, women and agriculture, they could just be a tiny country in a city.

But, what about semi-sci-fi-stuff? You could make a big bowl/dome, place it over the city, and just suck the air out. Not a nice way to kill, but effective all the same. Or what about digging away the ground underneath the city, then blow whatever supports keeping it in place so it sinks into the ground. Fill up the now dug-inside-a-hole city with concrete, problem solved.

The issue with most things is (as some have already posted):

You may have limited time

You may need support from your own people

You may want to have the city be useful afterwards

You could solve some of it by simply building a completely new and identical city somewhere else, since you have plenty of money, and just ignore the other one. Or go live in space and shoot any rockets down they might develop over time. Maybe destabilise the planet's core and have it tear itself apart while you're at it.

If you want to keep everything as-is, and just get rid of the people in the city: biological or chemical are probably the best choices, unless a kind of radiation is available that will not remain there for long. Gas masks don't last forever, and you can't live in a bunker for all eternity, so eventually you'll get them all.

if time and civilian casualties are non-issues, then siege is definitely the way to go. just start by using drone aircraft to kill anyone that leaves the city. then use drones to fire-bomb any gardens visible from the air. the city will starve to death within weeks, perhaps months if they are seriously over-provisioned. bombing the water and power infrastructure will accelerate the situation substantially.

$\begingroup$I don't think you need to kill people leaving the city, just everyone entering it should be enough so no supplies come.$\endgroup$
– Paŭlo EbermannAug 4 '16 at 23:11

$\begingroup$anyone leaving the city could theoretically be a guerrilla fighter, moreover they could report damaging propaganda back to your citizens. that said, in real life, intentionally and casually killing foreign civilians tends to hurt domestic morale almost as much as casualties on the home team. but then, this is decidedly not a "real life" situation.$\endgroup$
– james turnerAug 5 '16 at 14:11

I'm surprised no one has thought of using a genetically modified virus to wipe out the garrison. Three days to kill off the population, three days for the virus to die off and you can use POWs to clean up the bodies afterwards!

$\begingroup$Hi and welcome to the Worldbuilding SE. Perhaps you could expand on your answer by explaining how the virus needs to be genetically modified and how to prevent the virus from being spread outside the city by its defenders. There is no mention in the question that the defenders are genetically distinct from the attackers in any way after all.$\endgroup$
– CyrusAug 3 '16 at 8:38

This would be a typical siege. Surround them and starve them out. Cut off all power, fuel, and water coming into the city. 4,000,000 people eat a lot of food, drink a lot of water, and produce a lot of sewage. Dig in and create your own defensive fortifications. If they come out of their city they'll be mauled.

Use your tech to maintain air superiority. Control the local roads. Blockade sea access. Shoot everything that moves per the Allies over Germany in WW2.

100,000 motivated hostiles is a huge number, especially in a city. A big, big bomb would be the simplest way to deal with this, but you don't want to do that. I'm guessing the city isn't really abandoned?

Starvation is a sure strategy, but it will take a while for all the Twinkies and pickles in Oppela to spoil.

Disease goes hand in hand with starvation. These things happen naturally to a city under siege. As long as our guys keep the vigil, their guys will die off or surrender.

We can do this with primitive tech, so we can put the best available stuff to finding all their supply tunnels and tracking smugglers. Better yet, contaminate the food that gets smuggled in.

But here's the thing: it sounds like our biggest liability is our own population. They don't want us in Oppela. They don't want our guys to get a hangnail taking the city. They are against us in spite of our best propaganda. Instead of taking Oppela, let's join forces with the rebels and conquer our own population!